In recent years, steels have become increasingly stronger, but this causes a problem of deterioration in workability. Thus, there is a strong demand for steels having improved machinability while maintaining the strength. Conventionally, in order to improve the machinability of the steel, elements such as S, Pb, and Bi have been added for improving the machinability. Pb and Bi improve the machinability and have a relatively small effect on the forging, but deteriorate properties related to strength such as impact properties.
Further, the element S forms a soft inclusion such as MnS under cutting environments, thereby improving the machinability. However, MnS has a size larger than Pb or other particles, and hence, is likely to form a source of stress concentration. In particular, when MnS is stretched through forging or rolling, this causes anisotropy in impact properties and the like, and mechanical properties become significantly weak in a specific direction. This anisotropy of mechanical properties has to be taken into consideration in the case of designing a steel structure. Thus, it is necessary to employ a technique for reducing the anisotropy in the mechanical properties in the case when S is added to the steel.
As described above, even if elements effective in improving the machinability are added, the impact properties deteriorate, and hence, it is difficult to achieve both the strength and the machinability at the same time. Further, in recent years, from the viewpoint of environmental protection, there is a tendency to avoid using Pb. Thus, further technical innovations are required to achieve both the machinability and the strength of the steel.
Conventionally, there are several technical proposals for improving the machinability without deteriorating the strength. Patent Document 1 proposes a steel for a machine structure including: C: 0.05 to 1.2% (mass %, the same applies to the following elements); Si: 0.03 to 2%; Mn: 0.2 to 1.8%; P: 0.03% or lower (not including 0%); S: 0.03% or lower (not including 0%); Cr: 0.1 to 3%; Al: 0.06 to 0.5%; N: 0.004 to 0.025%; and O: 0.003% or lower (not including 0%), the steel further including Ca: 0.0005 to 0.02% and/or Mg: 0.0001 to 0.005%, and the steel including solute N: 0.002% or more, with a balance including iron and inevitable impurities, and the steel satisfying the following relationship of Expression (A).(0.1×[Cr]+[Al])/[O]≧150  Expression (A)where [Cr], [Al] and [O] represent amounts (mass %) of Cr, Al and O, respectively.
Patent Document 2 proposes a steel for a machine structure, the steel including: C: 0.01 to 0.7%; Si: 0.01 to 2.5%; Mn: 0.1 to 3%; S: 0.01 to 0.16%; and Mg: 0.02% or lower (not including 0%), the steel satisfying [Mg]/[S]≦7.7×10−3, in which, of sulfide-based inclusions observed in the steel, an average value of an aspect ratio of the sulfide-based inclusion having a long span of 5 μm or more is 5.2 or lower, and an average value of an aspect ratio of the sulfide-based inclusion having a long span of 50 μm or more is 10.8 or lower, and in which the steel satisfies a/b≦0.25, where a reference character a represents the number of sulfide-based inclusions having a long span of 20 μm or more, and a reference character b represents the number of sulfide-based inclusions having a long span of 5 μm or more.
Patent Document 3 proposes a steel for carburizing, the steel including: C: 0.12 to 0.22%; Si: 0.40 to 1.50%; Mn: 0.25 to 0.45%; Ni: 0.50 to 1.50%; Cr: 1.30 to 2.30%; B: 0.0010 to 0.0030%; Ti: 0.02 to 0.06%; Nb: 0.02 to 0.12%; and Al: 0.005 to 0.050%, with a balance substantially consisting of iron, in which a distance from a quenching end to a position having a hardness corresponding to 50% martensite in an end quenching test is 20 mm or more, and a component parameter H(H=106C(%)+10.8Si(%)+19.9Mn(%)+16.7Ni(%)+8.55Cr(%)+45.5Mo(%)+28) is 95 or less.